This invention relates in general to color imaging systems and, more particularly, to reducing edge effect (white gapping) between adjacent objects developed in different color planes in jump gap development systems.
Electrophotographic (EP) processes for producing a permanent image on media are well known and commonly used. In general, a common process includes: (1) charging a photoreceptor (optical photoconductor or OPC) such as a roller or continuous belt bearing a photoconductive material; (2) exposing the charged photoreceptor to imaging light (laser) that discharges the photoreceptor in select areas to define a latent electrostatic image on the photoreceptor; (3) presenting developer particles (toner) to the photoreceptor surface bearing the image so that the particles are transferred to the surface in the shape of the image; (4) transferring the particles in the shape of the image from the photoreceptor to the media; (5) fusing or fixing the particles in the shape of the image to the media; and (6) cleaning or restoring the photoreceptor for the next printing cycle. Many image forming apparatus, such as laser printers, copy machines, and facsimile machines, utilize this well known electrophotographic printing process.
Laser driven color printers and copiers employ toners that enable light to reflect off the page and to be directed back towards the eye. In general, such devices employ Cyan (C), Magenta (M) and Yellow (Y) toners as the principal component colors, from which other colors are created. Light passing through CMY toners has part of its color filtered out or absorbed by the toner such that the reflected light takes on the color of the toners that it passes through. In laser printers (and some copiers), a black (K) toner is also used which is opaque to light. When a printer receives image data from a host processor, the data is received in the form of either Red, Green and Blue (RGB) values, or CMYK values, or L*a*b* or some other conventional color space values. In any case, the received values are typically converted to CMYK values in order to achieve desired levels of color representation on the final printed document.
In view of the CMYK image values, many color EP devices, such as color laser printers, utilize a single pass or four-pass process to produce a full-color photoconductor, generally referred to herein as a multi color plane image processing cycle. For example, FIG. 1 is a block diagram depicting a conventional EP system wherein four developer modules 1, 2, 3 and 4 are arranged along a moving photoconductor surface/drum 5. Each developer module is allocated to the deposition of one of the CMY and K toners onto the moving photoconductor 5. A charging station (corona) 6 uniformly charges the photoconductor 5 and an exposure station (laser light) 7 selectively discharges the photoconductor in accordance with a color plane""s image data. The imaged photoconductor 5 then moves past the respective developer modules, with one developer module being moved into juxtaposition with the photoconductor to allow color toning of the discharged areas. The developed photoconductor then experiences a full rotation, is cleaned 8, charged again 6, and then exposed again in accordance with a next color plane""s data and again developed, using the next color developer. The procedure continues until four passes have occurred and a full color image is present on the photoconductor 5. Thereafter, the image is electrostatically transferred via a transfer roller 9 to a sheet media 11.
Alternatively, after development of any given color plane, that color plane""s image on the photoconductor surface may be indirectly transferred to the sheet media. Indirect transfer typically comprises transfer of the color plane""s image (or one or more planes of the color image) to an intermediate transfer (IT) member and then to the sheet media. Specifically, for example, upon each revolution of the photoconductor, one color plane will be imaged on the photoconductor and then immediately transferred to the IT member. In such indirect transfer, the IT member typically is large enough to hold the entire image plane at one time. Whether direct or indirect electrostatic image transfer occurs, the resultant image is subsequently fused to the sheet media.
In laser printers, the imaging element is generally a laser whose beam is image-wise scanned across the photoconductive belt or drum to produce a desired image. However, the laser optics (xe2x80x9cenginexe2x80x9d) portion of a color electrophotographic printer is both expensive and requires precise alignment to enable accurate super-position of cyan, magenta, yellow and black color planes to create a complete color image. Thus, to reduce the overall cost of such a color printer, only one laser imaging station is generally provided.
Conventional development stations typically are designed either for non-contact (xe2x80x9cjump-gapxe2x80x9d) development or contact development. In non-contact development, the conductive, cylindrical developer sleeve that carries the toner for electrostatic transfer to the photoconductor is separated from the photoconductor surface by a small gap, which is typically in the range of 200-500 microns. A cloud of toner particles is generated in the gap using an AC voltage (Vac) applied to the DC offset VDEV. In contact development, the developer sleeve rotates against the photoconductor surface. Toner is typically applied to the surface of the rotating sleeve, which then rotates the toner between the sleeve and the photoconductor, and a bias voltage is applied to the sleeve. In both non-contact and contact development, various transport and metering components may be used to apply toner to or near the sleeve surface (i.e., from a toner hopper or reservoir), including rollers, augers, paddles, blades or mixers, for example.
Jump-gap development is more susceptible to fringe effects and gaps between color fields than is contact development. These fringe (or edge) effects and gaps (also referred to as xe2x80x9cwhite gappingxe2x80x9d) appear as blurred (or white) image edges and result from imprecise toner development caused by lateral electric field effects between exposed and unexposed areas on the photoconductor surface. In the multiple-development systems of color printers, the fringe effects and gaps of jump-gap development may be magnified, depending on the development and transfer means employed in the process.
More specifically, edge effect or white gapping in a jump-gap development system is caused by the recession of non saturated (i.e., lighter or not fully exposed) colors away from the exposed edge. The recession is caused by the effect of the very negative OPC potential in the non imaged (unexposed or white) area that is edge adjacent to the imaged (exposed) region. At these edges, the development of the toner is inhibited and recedes away from the unexposed white area. This recession from the edge is particularly visible when the edge of the light region butts up against an object of a different color. The recession appears as a white gap between the light region and any object which is supposed to butt tightly against it and that is formed using a color plane other than the color plane of the light region. This white gap around the object can be very visible and results in customer dissatisfaction.
Although contact development may exhibit the benefit of increased image sharpness, it can be mechanically more complicated than jump-gap development in a multi-color process and has also its own associated xe2x80x9cghostingxe2x80x9d effects and problems. This complication is due to the fact that: (i) the individual development stations for each color must be engaged and disengaged from the photoconductor surface to affect the contact development of the individual color planes, and that (ii) multiple surfaces contact the toner in the development process which can lead to charging and background problems.
Prior solutions aimed at reducing the edge effect in jump-gap development systems have included increasing the strength of the DC field relative to the AC field. Although this results in less effect between adjoining areas, it can cause other serious side affects such as development instability over life. Another proposed solution has been to reduce the edge effect around black characters by using a process black neutral axis. Namely, with normal process black treatment (containing 30-40% of the underlying CMY colors) it is possible to reduce the edge effects at the cost of character acuity for black text. The character acuity decreases due to increased scatter brought on by greater toner pile height and also by mis-registration errors which can cause a character to appear as a double image made of the different color planes. However, this solution only works relative to black characters.
Accordingly, an object of the present invention is to reduce white gapping in color jump-gap imaging systems.
According to principles of the present invention in a preferred embodiment, a method for reducing edge effect (white gapping) between objects formed on a photoconductive member of an imaging device includes, during processing of a given color plane, under exposing (partially exposing) the photoconductive member for image data that specifies a color to be developed on at least one color plane but specifies no color to be developed on the given color plane.
According to further principles, under exposing occurs relative to a development threshold for the image processing of the given color plane. As such, the under exposing does not enable development of the image data during image processing of the given color plane. Additionally, the under exposing occurs relative to a high frequency halftone screen for minimizing contamination (or unwanted development) during the partial exposure. A half tone screen switching process enables appropriate halftone exposure for objects embodying the given color plane, and enables the necessary under exposure (utilizing the high frequency halftone screen) for objects not embodying the given color plane.
By partially exposing objects that do not include a color to be developed on the color plane currently being processed, and by normally exposing adjacent objects that include a color to be developed on the current color plane, the lateral electric field effects between the objects is reduced and, thus, toner development for the given color plane is more edge precise for reduced white gapping between the objects.
Other objectives, advantages, and capabilities of the present invention will become more apparent as the description proceeds.